Methods for producing epoxides include a chlorohydrin method, a co-oxidation method, a direct oxidation method, and the like. In the chlorohydrin method, a large amount of chlorine-containing sewage, which is detrimental to the environment, is produced after the reaction. The co-oxidation method is complex, and generates various joint products. The direct oxidation method can be classified into an oxygen direct oxidation method and a peroxide direct oxidation method. In the oxygen direct oxidation method for producing an epoxide, pure oxygen is directly introduced as a reactant. The oxygen direct oxidation method is simple, and does not produce any intermediate products. However, the selectivity of the product obtained by the oxygen direct oxidation method is low. In view of the aforesaid drawbacks, the peroxide direct oxidation method using a peroxide as an oxidant to carry out epoxidation is now widely used. In the peroxide direct oxidation method, a titanium-silicon molecular sieve is used as a catalyst which can be easily separated from the product after the reaction. The peroxide direct oxidation method does not harm the environment or consume a large amount of oxygen, but the selectivity and yield of epoxide are yet to be increased.
Usually, a crystalline titanium-silicon molecular sieve is prepared by a hydrothermal method, as mentioned in U.S. Pat. No. 5,290,533, U.S. Pat. No. 5,888,471, U.S. Pat. No. 5,977,009, U.S. Pat. No. 6,042,807, U.S. Pat. No. 6,083,864, U.S. Pat. No. 6,329,537, U.S. Pat. No. 6,972,337 or U.S. Pat. No. 7,288,237, for example. If alkali metal ions or alkaline earth metal ions are present during the preparation of a titanium-silicon molecular sieve in the hydrothermal method, an undesirable crystalline phase is formed. This lowers the catalytic activity of the titanium-silicon molecular sieve. Hence, the presence of these metal ions in the reaction solution should be avoided.
Therefore, it is an urgent issue to develop a method for producing an epoxide with increased raw material conversion rate, product selectivity and yield.